In exchanges of a telecommunication network which transmits and receives pulse code modulated signals (PCM), it is necessary to transmit the PCM signals to or from other exchanges by means of particularly adapted apparatus including a coupler. It is a known practice (see "Informationen Fernsprech-Vermittlungstechnik" 5, 1969, No. 1, pages 48-59) to allocate a PCM coupler to the PCM lines entering and leaving a PCM exchange. Each PCM coupler may also be allocated to a plurality of incoming and outgoing lines, the individual couplers in such case being interconnected via a separate connecting coupling field (see "Fernmeldetechnik" 9, 1969, No. 5, pages 147-152).
To check the operational reliability of a PCM coupler, a commonly used technique (see West German Pat. No. P 20 32 363.1) is to transmit a check word to an incoming PCM line within each pulse frame of the incoming PCM lines. That is, the check word occurs each time in the course of the duration of a specific time division, and each time on an outgoing PCM line of the PCM coupler when the latter functions properly. It is likewise conventional, in monitoring adjusting devices for adjusting coupling-point switches of a PCM coupler, to write monitoring codes into separate pairs of leads intermediate relay banks associated with adjoining exchanges in the course of the duration of the time slots of communications of non-assignable time channels (see West German Pat. No. P 20 48 115.6). The monitoring codes are compared with control signals which are generated by an interoffice timing generator at corresponding instants within the respective fixed pulse frame. If the signals that are being compared with each other do not coincide, error-detecting signals are generated which can be employed for correcting errors.
These known solutions for monitoring a PCM coupler require complex and expensive special apparatus which, to a large extent, can only be utilized for the monitoring operation itself.
It is, therefore, an object of this invention to provide means for monitoring the operational reliability of PCM couplers without the use of elaborate equipment.